Conventional thermosetting coatings are coatings which use as a crosslinking agent a melamine such as an alkyd melamine, an acrylic melamine or an epoxy melamine. However, these coatings have the problem of bad smelling caused by the melamine resin remains to be solved.
In order to solve these problems, the present inventors found a crosslinking system of a polyol resin and a hydrolyzable silyl group-containing resin which is quite different from a conventional crosslinking system of a polyol resin and a melamine resin, and already filed a patent application (see Japanese Unexamined Patent Publication 1-141952).
However, when such a mixture of the polyol resin and the hydrolyzable silyl group-containing polymer is used without a curing catalyst, the curing speed is slow at room temperature or under heating at a relatively low temperature. Accordingly, it is required to heat the mixture at a high temperature when it is desired to coat and cure at a high speed, requiring consumption of a large amount of energy.
Such disadvantages can be, generally, improved by admixing a curing catalyst just before the use of the coating composition, whereby the curing speed of the coating film is increased even at a relatively low temperature. However, after admixing the curing catalyst once, such compositions (which are generally called a "two component composition"), which are used as a paint, coating agent, adhesive, secant, coupling agent and the like, cannot be stored, since the compositions are cured in a short period of time. Accordingly, the unused portion of the coating composition comes to no use.
When it is desired to form a film utilizing a siloxy-crosslinking owing to a reaction of hydroxyl group of an acrylic resin having a hydroxyl group with an alkoxysilyl group of an alkoxysilyl group-containing acrylic copolymer, a composition is required to contain an acrylic resin having enough hydroxyl groups to sufficiently crosslink. In such a case, the hydroxyl groups and the alkoxysilyl groups are gradually reacted even in absence of a catalyst, thus resulting in gelation. Much less, it is technically difficult to stably store the composition containing a curing catalyst over several months.